"sending tarball" takes a long time even when the image already exists

When I build an image which already exists (because of a previous build on the same engine with 100% cache hit), the builder still spends a lot of time in "sending tarball". This causes a noticeable delay in the build. Perhaps this delay could be optimized away in the case of 100% cache hit?

For example, when building a 1.84GB image with 51 layers, the entire build is 9s, of which 8s is in "sending tarball" (see output below).

It would be awesome if fully cached builds returned at near-interactive speed!

 => [internal] load build definition from Dockerfile          0.0s
 => => transferring dockerfile: 2.53kB                        0.0s
 => [internal] load .dockerignore                             0.0s
 => => transferring context: 2B                               0.0s
 => [internal] load metadata for docker.io/library/alpine:la  1.0s
 => [1/51] FROM docker.io/library/alpine@sha256:6a92cd1fcdc8  0.0s
 => => resolve docker.io/library/alpine@sha256:6a92cd1fcdc8d  0.0s
 => CACHED [2/51] RUN apk update                              0.0s
 => CACHED [3/51] RUN apk add openssh                         0.0s
 => CACHED [4/51] RUN apk add bash                            0.0s
 => CACHED [5/51] RUN apk add bind-tools                      0.0s
 => CACHED [6/51] RUN apk add curl                            0.0s
 => CACHED [7/51] RUN apk add docker                          0.0s
 => CACHED [8/51] RUN apk add g++                             0.0s
 => CACHED [9/51] RUN apk add gcc                             0.0s
 => CACHED [10/51] RUN apk add git                            0.0s
 => CACHED [11/51] RUN apk add git-perl                       0.0s
 => CACHED [12/51] RUN apk add make                           0.0s
 => CACHED [13/51] RUN apk add python                         0.0s
 => CACHED [14/51] RUN apk add openssl-dev                    0.0s
 => CACHED [15/51] RUN apk add vim                            0.0s
 => CACHED [16/51] RUN apk add py-pip                         0.0s
 => CACHED [17/51] RUN apk add file                           0.0s
 => CACHED [18/51] RUN apk add groff                          0.0s
 => CACHED [19/51] RUN apk add jq                             0.0s
 => CACHED [20/51] RUN apk add man                            0.0s
 => CACHED [21/51] RUN cd /tmp && git clone https://github.c  0.0s
 => CACHED [22/51] RUN apk add go                             0.0s
 => CACHED [23/51] RUN apk add coreutils                      0.0s
 => CACHED [24/51] RUN apk add python2-dev                    0.0s
 => CACHED [25/51] RUN apk add python3-dev                    0.0s
 => CACHED [26/51] RUN apk add tar                            0.0s
 => CACHED [27/51] RUN apk add vim                            0.0s
 => CACHED [28/51] RUN apk add rsync                          0.0s
 => CACHED [29/51] RUN apk add less                           0.0s
 => CACHED [30/51] RUN pip install awscli                     0.0s
 => CACHED [31/51] RUN curl --silent --location "https://git  0.0s
 => CACHED [32/51] RUN curl https://dl.google.com/dl/cloudsd  0.0s
 => CACHED [33/51] RUN curl -L -o /usr/local/bin/kubectl htt  0.0s
 => CACHED [34/51] RUN curl -L -o /usr/local/bin/kustomize    0.0s
 => CACHED [35/51] RUN apk add ruby                           0.0s
 => CACHED [36/51] RUN apk add ruby-dev                       0.0s
 => CACHED [37/51] RUN gem install bigdecimal --no-ri --no-r  0.0s
 => CACHED [38/51] RUN gem install kubernetes-deploy --no-ri  0.0s
 => CACHED [39/51] RUN apk add npm                            0.0s
 => CACHED [40/51] RUN npm config set unsafe-perm true        0.0s
 => CACHED [41/51] RUN npm install -g yarn                    0.0s
 => CACHED [42/51] RUN npm install -g netlify-cli             0.0s
 => CACHED [43/51] RUN apk add libffi-dev                     0.0s
 => CACHED [44/51] RUN pip install docker-compose             0.0s
 => CACHED [45/51] RUN apk add mysql-client                   0.0s
 => CACHED [46/51] RUN (cd /tmp && curl -L -O https://releas  0.0s
 => CACHED [47/51] RUN apk add shadow sudo                    0.0s
 => CACHED [48/51] RUN echo '%wheel ALL=(ALL) NOPASSWD: ALL'  0.0s
 => CACHED [49/51] RUN useradd -G docker,wheel -m -s /bin/ba  0.0s
 => CACHED [50/51] RUN groupmod -o -g 999 docker              0.0s
 => CACHED [51/51] WORKDIR /home/sh                           0.0s
 => exporting to oci image format                             8.8s
 => => exporting layers                                       0.3s
 => => exporting manifest sha256:69088589c4e63094e51ae0e34e6  0.0s
 => => exporting config sha256:65db1e1d42a26452307b43bc5c683  0.0s
 => => sending tarball                                        8.3s
 => importing to docker                                       0.1s

Reproduce the reported build using the OCI image export path and observe the “sending tarball” step during a 100% cache-hit build. Trace the export and Docker import flow to identify why the existing image is still transferred. Done means fully cached builds avoid the unnecessary delay while preserving the exported image.